Add user options to control pgloader batch behaviour.

The new WITH options allows the user to set values for the dynamic
variables *copy-batch-rows*, *copy-batch-size* and *concurrent-batches*.
That's needed in case like in issue #16 even with the batch size
defaulting to what looks like a proper setup.

In a longer term a review of the pgloader memory usage should be done
seriously, the numbers being way higher than the batch sizes we do setup
here.